Ionization detectors measure radiation by means of the number of charge carriers set free in the detector. The active detection volume is typically arranged between two electrodes. Ionizing radiation produces free electrons and holes within the detector material, often a semiconductor. The number of electron-hole pairs is proportional to the energy transmitted by the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band. Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured. The holes travel in the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is mostly independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be determined.
Radiation detectors using simple planar electrodes and based on ionization measurements often suffer from poor collection of charge carriers. For example, positive charge carriers (holes) may migrate through the detector medium at a much slower rate than negative charge carriers (electrons). As a result, such detectors produce signals that vary in amplitude depending on the location within the detector at which incident radiation interacts with the detector medium. Such detectors include, but are not limited to, semiconductor detectors, liquid ionization detectors, and gas ionization detectors.
In a simple planar electrode ionization detector, full-area electrodes are formed on two opposing faces of the detector medium. A bias voltage applied across the two electrodes provides an electric field to separate and collect the charge carriers that are created by the absorption of radiation in the detector medium. Induced charge signal on one of the electrodes due to the motion of carriers provides a measure of the energy of the radiation. Incomplete charge collection due to carrier trapping or slow carrier transport results in reduced signals, which vary in strength depending on the depth of radiation interaction. This degrades the energy resolution of the detector.
Several detector structures make use of the concept of multiple electrode collection to minimize the detrimental effects of charge trapping. Rather than trying to force better collection of the holes, a number of practitioners have taken the approach to design devices in which the signal due to the holes is relatively insignificant.
Ramo's Theorem explains the effect on the spectrum that occurs in devices with uniform and with non-uniform electric fields. In a planar device with full area electrodes, the electric field is uniform throughout the device. A device with a very small point electrode will have a field that increases rapidly close to that electrode. Thus, when the electron is close to the electrode, it will pass through a higher electric field and generate a larger signal in the electrode. The simple planar device has a uniform field and in the general case where the electrodes are large compared to the volume occupied by the electric charge the response at all points in the device is linear. Thus, all of the electrons and all of the holes contribute to the detected signal.